The present invention relates to an image forming apparatus performing image forming by superimposing images of at least two colors, an image forming system, and a color shift correction method.
In recent years, full-color compatible image forming apparatuses have been widely prevalent. An example of such a full-color compatible image forming apparatus is an apparatus that forms a color image by superimposing images (toner images) individually formed by individual colors of cyan (C), magenta (M), and yellow (Y). As one such image forming system, a tandem system that forms toner images of individual colors of cyan (C), magenta (M), yellow (Y), and black (K) using individual image forming units is known. In the tandem system, the image forming units of the individual colors of CMYK are arrayed in the moving direction of the transfer belt, the timings of the formation of the toner images of the individual image forming units are controlled so that the toner images formed in the individual image forming units are transferred to the same position in the transfer belt.
In this way, in an image forming apparatus where one image is formed by superimposing images individually formed with respect to individual colors, when a shift (a so-called color shift) occurs when the images of individual colors are superimposed, the quality of a formed image is significantly reduced. Examples of such a color shift include a color shift caused by: the images of individual colors being relatively shifted in a direction (a main scanning direction) perpendicular to the moving direction of the transfer belt; the images of the individual colors being relatively shifted in a direction (a sub-scanning direction) parallel to the moving direction of the transfer belt; by the widths (magnifications) of the images of the individual colors being different; and by scan lines in the main scanning direction used for forming the images of the individual colors being inclined.
For example, a color shift caused by a shift in the main scanning direction or a shift in the sub-scanning direction may be corrected by adjusting an image writing start timing via an exposure device in the main scanning direction or the sub-scanning direction. In addition, a color shift caused by a magnification may be corrected by adjusting the frequency of the clock signal used for image writing via the exposure device. Furthermore, the inclination of the scan line may be corrected by adjusting the attachment position of the exposure device or the image carrier.
Even if color shift correction was implemented at the time of factory shipment, when a member relating to image forming has been attached or removed after that, in some cases such a color shift as described above occurs owing to a subtle change in an attachment position or vibration at the time of an attachment operation. Therefore, in the situation where the detachment of a member relating to image forming was detected, color shift correction is implemented when the corresponding member has been attached.
In many cases, image forming apparatuses in recent years have been configured so as to automatically implement the above-mentioned color shift correction. However, members configuring the image forming units of the individual colors are not completely identical to one another, and have dimensional variations caused by process variations even if they are the same type of member. In addition, such a process variation also exists within the same member. Therefore, for example, in rotary members or the like such as photoreceptor drums configuring the image forming units of individual colors, technically the rotation radius of a rotating body is not uniform, or a rotational fluctuation occurs that is caused by a driving fluctuation in a drive system for the rotating body.
A minute variation such as the non-uniformity of the rotation radius, or rotational fluctuation, is inherent in the image forming unit of each color of CMYK, and occurs with having no correlation with the other image forming units. Therefore, even when the color shift correction has been automatically implemented, when taking into account two specified colors, in some cases a color shift has occurred the degree of which influences an image quality. In order to solve such a color shift, fine adjustments become necessary.
As a method for such fine adjustment, the image forming apparatus can be caused to output a chart for such fine adjustment, and color shift correction is subjected to the fine adjustment based on the output result of the corresponding chart. In this method, as illustrated in FIG. 12, as a chart 1200 for fine adjustment, image data is used where a pattern 1201 that includes a plurality of parallel lines including a reference color (for example, K) and having equal pitches and equal line widths and a pattern 1202 that includes a plurality of parallel lines including an adjustment color (for example, C) and having equal pitches and equal line widths are adjacent to each other. Here, the pitch of the parallel lines of the adjustment color is wider by one dot than the pitch of the parallel lines of the reference color, and the line widths of the parallel lines of the two colors are identical to each other.
The image forming apparatus is caused to output this chart for adjustment, a portion where the lines of the two colors coincide with each other is read, and the adjustment amount (an adjustment value) is decided based on the result thereof. For example, when the pitch of the parallel lines of the reference color is five dots, and the line width of each parallel line is one dot (the one dot is, for example, 1/600 inches square), the lines of two colors coincide with each other every six lines in the reference color. If the position where the lines of two colors coincide with each other is such a position as designed, no color shift occurs. On the other hand, if the position where the lines of two colors coincide with each other is different from the design position, a color shift occurs. For example, when, in the reference color, the position where the lines of two colors coincide with each other is shifted by two lines from the design position, the adjustment color generates a two-dot color shift with respect to the reference color.
However, in the above-mentioned chart for fine adjustment, as the print area of the chart for fine adjustment, used for obtaining the adjustment value, a relatively large area becomes necessary. For example, in the above-mentioned example, as the area for forming the parallel lines of the reference color, a width of about 30 mm becomes necessary. In other words, it is only possible to obtain one color shift adjustment amount from the width of about 30 mm.
As described above, since, in the image forming unit of each color of CMYK, the color shift occurs having no correlation with the other image forming units, the occurrence state of a color shift is different in the different position on a sheet of paper. In addition, even in the same position on a sheet of paper, for example, the occurrence state of a color shift between CK and the occurrence state of a color shift between MK are different from each other. Therefore, so as to execute the fine adjustment of color shift correction over the entire sheet of paper, it is desirable that the color shift adjustment value with respect to each adjustment color (for example, each of CMY) is acquired.
However, in the above-mentioned chart for fine adjustment, there has been a problem in that the number of charts locatable on a sheet of paper is limited due to the restriction of the size thereof and it is difficult to acquire adjustment values with respect to each adjustment color throughout the entire sheet of paper. In addition, even if a large number of charts for fine adjustment are locatable on a sheet of paper, there is a problem that, so as to acquire adjustment values, it is necessary to confirm the position where the lines of two colors coincide with each other in each chart for fine adjustment and the large amount of work is taken. Therefore, the above-mentioned method has not been more than satisfactory, as a fine adjustment method for color shift correction.